Several issues with XPS 13, Ubuntu 16

Hello, I've got a few things that are bugging me with this laptop, and still furloughed, so I still have a little time to deal with them all, I hope. I'm going to list them in order of how annoying they are :~)

#1. Sound! I just recently got an HDMI cable to hook up to the TV. The picture was fine, but the sound not so much. It played OK sometimes, and then not, and I'm pretty sure that I hadn't changed any setting in the meantime. I ended up fiddling around a lot with pavucontrol and alsa mixer, which didn't really fix anything. In fact, I ended up yesterday with this extra Dummy Output knocking out *every* other kind of output. So I actually had to do a factory reset of the audio settings, following the instructions at the very end of this post. So fortunately I do at least have the onboard sound back, and actually I just connected with HDMI and got sound, but I very much doubt that the bigger issue has been sorted and am not expecting that to be stable.

The story with connecting bluetooth speakers or headphones is very similar to the experience that I've had with HDMI. They did occasionally connect at first, but were certainly not reliable. Of course I just gave up after a while, but did try yesterday, and sure enough no good. I have Bluetooth Manager installed, but that doesn't seem to make any difference. Like most people, I guess, I'm not some g reat Linux expert, so I am pretty much at the end of what I think I can do about this. Is it just a fundamental issue with this laptop, or can it be fixed? Would upgrading to 18 or 20 resolve the problem? Which takes me to.......

#2. Upgrading! I've tried, but failed. I followed all the steps on this page and at the last step - sudo do-release-upgrade - I get this:

Checking for a new Ubuntu release
No new release found.
FAIL: 1

So.... What is going on there? Does Dell block the update? I've tried using Software Updater, as described here, and can't find any Ubuntu upgrade option. Nor do I see the possibility to choose "Any version" as opposed to LTS versions. Obviously I'm going to get quite irate if I can't upgrade *at all*.

#3. Delayed shutdown/restart. When I select "shutdown", instead of getting the option to shutdown or restart, there is a sometimes very slow logout after which I have to shut down or restart from the log on screen. Surely that can't be right????

#4. The Super key doesn't work properly. Weird, actually. Super+Space does exactly what it's supposed to do, but Super on its own doesn't do anything. It's supposed to bring up the activities menu, right? Also, for example, Super+L doesn't lock the screen.

I suppose that there might be some other issues that I can't think of right now, but I also suppose that if I can't think of them right now, they can't be toooooo important. Thanks in advance